Conventionally, a microphone is applied to voice input apparatuses such as voice communication apparatuses like a mobile phone, a transceiver and the like, information process apparatuses like a voice identification system and the like that use a technology for analyzing an input voice, or a record apparatus (e.g., see patent documents 1 and 2). The microphone unit has a function that transduces an input voice into an electrical signal and outputs the electrical signal.
FIG. 17 is a schematic sectional view showing a structure of a conventional microphone unit 100. As shown in FIG. 17, the conventional microphone unit 100 includes: a base board 101; an electrical acoustic transducer portion 102 that is mounted on the base board 101 and transduces a sound pressure into an electrical signal; an electrical circuit portion 103 that is mounted on the base board 101 and applies an amplification process and the like to the electrical signal obtained by the electrical acoustic transducer portion 102; and a cover 101 that protects the electrical acoustic transducer portion 102 and the electrical circuit portion 103 mounted on the base board 101 from dust and the like. The cover 104 is provided with a sound hole (through-hole) 104a and an external sound is guided to the electrical acoustic transducer portion 102.
Here, in the microphone unit 100 shown in FIG. 17, the electrical acoustic transducer portion 102 and the electrical circuit portion 103 are mounted by using a die bonding technology and a wire bonding technology.
In such microphone unit 100, as described in the patent document 1, it is general that the cover 104 is formed of a material that has a electromagnetic shield. function such that the electrical acoustic transducer portion 102 and the electrical circuit portion 103 are not subjected to an influence of external electromagnetic noise. Besides, as described in the patent document 2, for electromagnetic noise measures at the electrical acoustic transducer portion 102 and the electrical circuit portion 103, the base board 101 is formed of a multiple layer by means of an insulating layer and an electrically conductive layer such that the electrically conductive layer is embedded in the insulating layer, so that electromagnetic shielding is performed.